The long term theme of this research is to identify and understand the genetic and evolutionary factors that determine the distribution and abundance of transposable elements among species. We propose to exploit novel features of the Drosophila transposable element mariner in order to define genetic factors that control transposition and excision. The mariner element is active in somatic cells, so that excision, and to lesser extent transposition, can be followed phenotypically. The phenomenon of greatest immediate interest is inherited somatic mosaicism, in which an autonomous copy of mariner results in a high frequency of excision of all mariner elements, including a reporter element inserted in the white-peach allele. Thus, the mariner system is the first in Drosophila that provides the kinds of phenotypic indicators that have proven so powerful in the analysis of transposable elements in maize. The proposed genetic studies include four specific aims: Aim 1. Analysis of the autonomous Mos (mosaic) factor that causes inherited somatic mosaicism in order to identify the molecular mechanism of the high rates of mariner excision. Aim 2. Cloning and analysis of two additional, independently derived, cases of inherited somatic mosaicism, and determination of the mechanism of the distinct mosaic phenotypes "high" (early excision events, large patches) versus "low" (late events, small patches). Aim 3. Characterization of the insertional specificity and excision properties of the mariner element. Aim 4. Introduction of the white-peach allele and the Mos factor in to the genome of D. melanogaster by means of P-element transformation to allow detailed genetic analysis in this species and to develop the system for use in other types of genetic studies (e.g., transposon tagging, mosaic production, alternative system for transformation). The mariner element is also ideally suited for evolutionary studies because functional copies appear to be concentrated in the Drosophila melanogaster species subgroup, in which the phylogenetic relationships among species are well established. Evolutionary aspects of mariner evolution will be pursued in the fifth specific aim: Aim 5. To determine whether DNA sequences of mariner among related Drosophila species support the model of stochastic loss or horizontal transfer.